Galactic Center

{{About|the astronomical point in the Milky Way|the general concept|Galaxy#Center|the book series|Galactic Center Saga}}

The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy.{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=An Electrifying View of the Heart of the Milky Way – A new radio-wave image of the center of our galaxy reveals all the forms of frenzy that a hundred million or so stars can get up to. |url=https://www.nytimes.com/2022/01/31/science/milky-way.html |date=31 January 2022 |work=The New York Times |access-date=1 February 2022 }}{{cite journal|author=Heywood, I.|display-authors=etal |date=28 January 2022|title=The 1.28 GHZ MeerKAT Galactic Center Mosaic|journal=The Astrophysical Journal |volume=925 |issue=2 |page=165 |doi=10.3847/1538-4357/ac449a |arxiv=2201.10541|bibcode=2022ApJ...925..165H |s2cid=246275657 |doi-access=free }} Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact radio source which is almost exactly at the galactic rotational center.{{clarification needed|reason=Is the Center within its event horizon or only its sphere of influence?|date=April 2024}} The Galactic Center is approximately {{convert|8|kpc|ly}} away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula.

There are around 10 million stars within one parsec of the Galactic Center, dominated by red giants, with a significant population of massive supergiants and Wolf–Rayet stars from star formation in the region around 1 million years ago. The core stars are a small part within the much wider central region, called galactic bulge.

Discovery

File:Infrared-visible light comparison of VISTA’s gigapixel view of the centre of the Milky Way.ogv on Paranal in Chile and compares it with the view in visible light. Because VISTA has a camera sensitive to infrared light, it can see through much of the dust blocking the view in visible light, although many more opaque dust filaments still show up well in this picture.]]

Because of interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, ultraviolet, or soft (low-energy) X-ray wavelengths. The available information about the Galactic Center comes from observations at gamma ray, hard (high-energy) X-ray, infrared, submillimetre, and radio wavelengths.

Immanuel Kant stated in Universal Natural History and Theory of the Heavens (1755) that a large star was at the center of the Milky Way galaxy, and that Sirius might be the star.{{Cite magazine |last=Ley |first=Willy |date=August 1965 |title=The Galactic Giants |department=For Your Information |url=https://archive.org/stream/Galaxy_v23n06_1965-08#page/n129/mode/2up |magazine=Galaxy Science Fiction |pages=130–142 }} Harlow Shapley stated in 1918 that the halo of globular clusters surrounding the Milky Way seemed to be centered on the star swarms in the constellation of Sagittarius, but the dark molecular clouds in the area blocked the view for optical astronomy.{{cite journal |bibcode=1918ApJ....48..154S |title=Studies based on the colors and magnitudes in stellar clusters. VII. The distances, distribution in space, and dimensions of 69 globular clusters |journal=The Astrophysical Journal |volume=48 |page=154 |last=Shapley |first=H |year=1918 |doi=10.1086/142423|doi-access=free }}

In the early 1940s Walter Baade at Mount Wilson Observatory took advantage of wartime blackout conditions in nearby Los Angeles, to conduct a search for the center with the {{convert|100|in|cm|adj=on}} Hooker Telescope. He found that near the star Alnasl (Gamma Sagittarii), there is a one-degree-wide void in the interstellar dust lanes, which provides a relatively clear view of the swarms of stars around the nucleus of the Milky Way galaxy.{{cite journal |bibcode=1946PASP...58..249B |title=A Search for the Nucleus of Our Galaxy |journal=Publications of the Astronomical Society of the Pacific |volume=58 |issue=343 |page=249 |last=Baade |first=W |year=1946 |doi=10.1086/125835}} This gap has been known as Baade's Window ever since.{{cite journal |bibcode=1996A&A...310..771N |title=The galactic structure towards the Galactic Center. III. A study of Baade's Window: Discovery of the bar population? |journal=Astronomy and Astrophysics |volume=310 |page=771 |last1=Ng |first1=Y. K |last2=Bertelli |first2=G |last3=Chiosi |first3=C |last4=Bressan |first4=A |year=1996}}

At Dover Heights in Sydney, Australia, a team of radio astronomers from the Division of Radiophysics at the CSIRO, led by Joseph Lade Pawsey, used "sea interferometry" to discover some of the first interstellar and intergalactic radio sources, including Taurus A, Virgo A and Centaurus A. By 1954 they had built an {{convert|80|ft|m|adj=on}} fixed dish antenna and used it to make a detailed study of an extended, extremely powerful belt of radio emission that was detected in Sagittarius. They named an intense point-source near the center of this belt Sagittarius A, and realised that it was located at the very center of the Galaxy, despite being some 32 degrees south-west of the conjectured Galactic Center of the time.{{cite journal |bibcode=1955ApJ...121....1P |title=A Catalogue of Reliably Known Discrete Sources of Cosmic Radio Waves |journal=The Astrophysical Journal |volume=121 |page=1 |last=Pawsey |first=J. L |year=1955 |doi=10.1086/145957}}

In 1958 the International Astronomical Union (IAU) decided to adopt the position of Sagittarius A as the true zero coordinate point for the system of galactic latitude and longitude.{{cite journal |title=The new IAU system of galactic coordinates (1958 revision) |first1=A. |last1=Blaauw |last2=Gum |first2=C.S. |last3=Pawsey |first3=J.L. |last4=Westerhout |first4=G. |date=1960 |journal=Monthly Notices of the Royal Astronomical Society |volume=121 |issue=2 |pages=123–131 |bibcode=1960MNRAS.121..123B |doi=10.1093/mnras/121.2.123|doi-access=free }} In the equatorial coordinate system the location is: RA {{RA|17|45|40.04}}, Dec {{DEC|-29|00|28.1}} (J2000 epoch).

In July 2022, astronomers reported the discovery of massive amounts of prebiotic molecules, including some associated with RNA, in the Galactic Center of the Milky Way galaxy.{{cite news |last=Starr |first=Michelle |title=Loads of Precursors For RNA Have Been Detected in The Center of Our Galaxy |url=https://www.sciencealert.com/scientists-have-found-a-bunch-of-rna-precursors-in-the-galactic-center |date=8 July 2022 |work=ScienceAlert |access-date=9 July 2022 }}{{cite journal |author=Rivilla, Victor M. |display-authors=etal |title=Molecular Precursors of the RNA-World in Space: New Nitriles in the G+0.693-0.027 Molecular Cloud |date=8 July 2022 |journal=Frontiers in Astronomy and Space Sciences |volume=9 |doi=10.3389/fspas.2022.876870 |doi-access=free |arxiv=2206.01053 |bibcode=2022FrASS...9.6870R }}

Distance to the Galactic Center

File:Simulation of the X-shaped bulge of the Milky Way.ogv

The exact distance between the Solar System and the Galactic Center is not certain,{{Cite journal |title=Analysis of Determinations of the Distance between the Sun and the Galactic Center

|first=Zinovy M. |last=Malkin |s2cid=55662712 |arxiv=1301.7011 |journal=Astronomy Reports |volume=57 |issue=2 |pages=128–133 |date=February 2013 |doi=10.1134/S1063772913020078 |bibcode=2013ARep...57..128M |citeseerx=10.1.1.766.631}} Russian original {{cite journal |journal=Astronomicheskii Zhurnal |title=Об определении расстояния от Солнца до центра Галактики |last=Малкин |first=З. М. |year=2013 |volume=90 |issue=2 |pages=152–157 |language=ru |doi=10.7868/S0004629913020072}} although estimates since 2000 have remained within the range {{convert|24|-|28.4|kly|kpc|abbr=off|lk=on}}. The latest estimates from geometric-based methods and standard candles yield the following distances to the Galactic Center:

{{val|7.4|0.2|s=(stat) ± 0.2(syst)}} or {{val|7.4|0.3|u=kpc}} ({{val|24|1|p=≈|ul=kly}}){{cite journal |first1=Charles |last1=Francis |first2=Erik |last2=Anderson |s2cid=119235554 |title=Two estimates of the distance to the Galactic Centre |journal=Monthly Notices of the Royal Astronomical Society |date=June 2014 |volume=441 |issue=2 |pages=1105–1114 |doi=10.1093/mnras/stu631 |doi-access=free |bibcode=2014MNRAS.441.1105F |arxiv=1309.2629}}
{{val|7.62|0.32|u=kpc}} ({{val|24.8|1|p=≈|u=kly}}){{cite journal |last1=Eisenhauer |first1=F. |last2=Genzel |first2=R. |last3=Alexander |first3=T. |last4=Abuter |first4=R. |last5=Paumard |first5=T. |last6=Ott |first6=T. |last7=Gilbert |first7=A. |last8=Gillessen |first8=S. |last9=Horrobin |first9=M. |last10=Trippe |first10=S. |last11=Bonnet |first11=H. |last12=Dumas |first12=C. |last13=Hubin |first13=N. |last14=Kaufer |first14=A. |last15=Kissler-Patig |first15=M. |last16=Monnet |first16=G. |last17=Ströbele |first17=S. |last18=Szeifert |first18=T. |last19=Eckart |first19=A. |last20=Schödel |first20=R. |last21=Zucker |first21=S. |year=2005 |title=SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month |journal=The Astrophysical Journal |volume=628 |issue=1 |pages=246–259 |bibcode=2005ApJ...628..246E |doi=10.1086/430667 |arxiv=astro-ph/0502129|s2cid=122485461 }}
{{val|7.7|0.7|u=kpc}} ({{val|25.1|2.3|p=≈|u=kly}}){{cite journal |last1=Majaess |first1=D.J. |last2=Turner |first2=D.G. |last3=Lane |first3=D.J. |s2cid=14316644 |year=2009 |title=Characteristics of the Galaxy according to Cepheids |journal=MNRAS |volume= 398 |issue=1 |pages=263–270 |bibcode=2009MNRAS.398..263M |doi=10.1111/j.1365-2966.2009.15096.x |doi-access=free |arxiv=0903.4206}}
7.94 or {{val|8.0|0.5|u=kpc}} ({{val|26|1.6|p=≈|u=kly}}){{cite journal |last=Reid |first=Mark J. |title=The distance to the center of the Galaxy |journal=Annual Review of Astronomy and Astrophysics |date=1993 |volume=31 |issue=1 |pages=345–372 |bibcode=1993ARA&A..31..345R |doi=10.1146/annurev.aa.31.090193.002021}}{{cite journal |author=Eisenhauer, F. |author2=Schödel, R. |author3=Genzel, R. |author4=Ott, T. |author5=Tecza, M. |author6=Abuter, R. |author7=Eckart, A. |author8=Alexander, T. |s2cid=16425333 |title=A Geometric Determination of the Distance to the Galactic Center |journal=The Astrophysical Journal |date=2003 |volume=597 |issue=2 |pages=L121–L124 |bibcode=2003ApJ...597L.121E |arxiv=astro-ph/0306220 |doi=10.1086/380188}}{{cite journal |author=Horrobin, M. |author2=Eisenhauer, F. |author3=Tecza, M. |author4=Thatte, N. |author5=Genzel, R. |author6=Abuter, R. |author7=Iserlohe, C. |author8=Schreiber, J. |author9=Schegerer, A. |author10=Lutz, D. |author11=Ott, T. |author12=Schödel, R. |title=First results from SPIFFI. I: The Galactic Center |journal=Astronomische Nachrichten |date=2004 |volume=325 |issue=2 |pages=120–123 |url=http://www.mpe.mpg.de/SPIFFI/preprints/first_result_an1.pdf |doi=10.1002/asna.200310181 |bibcode=2004AN....325...88H |archive-url=https://web.archive.org/web/20070621072120/http://www.mpe.mpg.de/SPIFFI/preprints/first_result_an1.pdf |archive-date=2007-06-21 }}
{{val|7.98|0.15|s=(stat) ± 0.20(syst)}} or {{val|8.0|0.25|u=kpc}} ({{val|26|0.8|p=≈|u=kly}}){{cite arXiv |eprint=1202.6128 |title=The current best estimate of the Galactocentric distance of the Sun based on comparison of different statistical techniques |last=Malkin |first=Zinovy |year=2012|class=astro-ph.GA }}
{{val|8.33|0.35|u=kpc}} ({{val|27|1.1|p=≈|u=kly}}){{cite journal |last=Gillessen |first=S. |author2=Eisenhauer |author3=Trippe |author4=Alexander |author5=Genzel |author6=Martins |author7=Ott |s2cid=1431308 |title=Monitoring Stellar Orbits Around the Massive Black Hole in the Galactic Center |journal=The Astrophysical Journal |volume=692 |issue=2 |pages=1075–1109 |date=2009 |doi=10.1088/0004-637X/692/2/1075 |bibcode=2009ApJ...692.1075G |arxiv=0810.4674}}
{{val|8.0|0.3|u=kpc}} ({{val|25.96|0.98|p=≈|u=kly}}){{cite journal |last=Camarillo|first=T. |author2=Mathur |author3=Mitchell|author4=Ratra|title=Median Statistics Estimate of the Distance to the Galactic Center |journal=Publications of the Astronomical Society of the Pacific |volume=130|issue=984 |date=2018 |page=024101 |doi=10.1088/1538-3873/aa9b26 |bibcode= 2018PASP..130b4101C |arxiv=1708.01310|s2cid=118936491 }}
{{val|8.7|0.5|u=kpc}} ({{val|28.4|1.6|p=≈|u=kly}}){{cite journal |last1=Vanhollebeke |first1=E. |last2=Groenewegen |first2=M. A. T. |last3=Girardi |first3=L. |title=Stellar populations in the Galactic bulge. Modelling the Galactic bulge with TRILEGAL |bibcode=2009A&A...498...95V |journal=Astronomy and Astrophysics |volume=498 |issue=1 |pages=95–107 |doi=10.1051/0004-6361/20078472 |date=April 2009 |arxiv=0903.0946|s2cid=125177722 }}
{{val|8.122|0.031|u=kpc}} ({{val|26.49|0.1|p=≈|u=kly}}){{cite journal |last1=Abuter |first1=R. |last2=Amorim |first2=A. |last3=Anugu |first3=N. |last4=Bauböck |first4=M. |last5=Benisty |first5=M. |last6=Berger |first6=J. P. |last7=Blind |first7=N. |last8=Bonnet |first8=H. |last9=Brandner |first9=W. |last10=Buron |first10=A. |last11=Collin |first11=C. |last12=Chapron |first12=F. |last13=Clénet |first13=Y. |last14=Foresto |first14=V. dCoudé u |last15=Zeeuw |first15=P. T. de |last16=Deen |first16=C. |last17=Delplancke-Ströbele |first17=F. |last18=Dembet |first18=R. |last19=Dexter |first19=J. |last20=Duvert |first20=G. |last21=Eckart |first21=A. |last22=Eisenhauer |first22=F. |last23=Finger |first23=G. |last24=Schreiber |first24=N. M. Förster |last25=Fédou |first25=P. |last26=Garcia |first26=P. |last27=Lopez |first27=R. Garcia |last28=Gao |first28=F. |last29=Gendron |first29=E. |last30=Genzel |first30=R. |last31=Gillessen |first31=S. |last32=Gordo |first32=P. |last33=Habibi |first33=M. |last34=Haubois |first34=X. |last35=Haug |first35=M. |last36=Haußmann |first36=F. |last37=Henning |first37=Th |last38=Hippler |first38=S. |last39=Horrobin |first39=M. |last40=Hubert |first40=Z. |last41=Hubin |first41=N. |last42=Rosales |first42=A. Jimenez |last43=Jochum |first43=L. |last44=Jocou |first44=L. |last45=Kaufer |first45=A. |last46=Kellner |first46=S. |last47=Kendrew |first47=S. |last48=Kervella |first48=P. |last49=Kok |first49=Y. |last50=Kulas |first50=M. |last51=Lacour |first51=S. |last52=Lapeyrère |first52=V. |last53=Lazareff |first53=B. |last54=Bouquin |first54=J.-B. Le |last55=Léna |first55=P. |last56=Lippa |first56=M. |last57=Lenzen |first57=R. |last58=Mérand |first58=A. |last59=Müler |first59=E. |last60=Neumann |first60=U. |last61=Ott |first61=T. |last62=Palanca |first62=L. |last63=Paumard |first63=T. |last64=Pasquini |first64=L. |last65=Perraut |first65=K. |last66=Perrin |first66=G. |last67=Pfuhl |first67=O. |last68=Plewa |first68=P. M. |last69=Rabien |first69=S. |last70=Ramírez |first70=A. |last71=Ramos |first71=J. |last72=Rau |first72=C. |last73=Rodríguez-Coira |first73=G. |last74=Rohloff |first74=R.-R. |last75=Rousset |first75=G. |last76=Sanchez-Bermudez |first76=J. |last77=Scheithauer |first77=S. |last78=Schöller |first78=M. |last79=Schuler |first79=N. |last80=Spyromilio |first80=J. |last81=Straub |first81=O. |last82=Straubmeier |first82=C. |last83=Sturm |first83=E. |last84=Tacconi |first84=L. J. |author84-link=Linda Tacconi|last85=Tristram |first85=K. R. W. |last86=Vincent |first86=F. |last87=Fellenberg |first87=S. von |last88=Wank |first88=I. |last89=Waisberg |first89=I. |last90=Widmann |first90=F. |last91=Wieprecht |first91=E. |last92=Wiest |first92=M. |last93=Wiezorrek |first93=E. |last94=Woillez |first94=J. |last95=Yazici |first95=S. |last96=Ziegler |first96=D. |last97=Zins |first97=G. |title=Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole |journal=Astronomy & Astrophysics |date=2018-07-01 |volume=615 |pages=L15 |doi=10.1051/0004-6361/201833718|arxiv=1807.09409 |bibcode=2018A&A...615L..15G |hdl=10871/35577 |s2cid=118891445 |hdl-access=free }}
{{val|8.178|0.013|s=(stat) ± 0.022(syst)}} kpc ({{val|26.67|0.1|p=≈|u=kly}}){{cite journal |author=R. Abuter |author2=A. Amorim |author3=M. Bauböck |author4=J. P. Berger |author5=H. Bonnet |author6=W. Brandner |author7=Y. Clénet |author8=V. Coudé du Foresto |author9=P. T. de Zeeuw |author10=J. Dexter |author11=G. Duvert |author12=A. Eckart |author13=F. Eisenhauer |author14=N. M. Förster Schreiber |author15=P. Garcia |author16=F. Gao |author17=E. Gendron |author18=R. Genzel |author19=O. Gerhard |author20=S. Gillessen |author21=M. Habibi |author22=X. Haubois |author23=T. Henning |author24=S. Hippler |author25=M. Horrobin |author26=A. Jiménez-Rosales |author27=L. Jocou |author28=P. Kervella |author29=S. Lacour |author30=V. Lapeyrère |author31=J.-B. Le Bouquin |author32=P. Léna |author33=T. Ott |author34=T. Paumard |author35=K. Perraut |author36=G. Perrin |author37=O. Pfuhl |author38=S. Rabien |author39=G. Rodriguez Coira |author40=G. Rousset |author41=S. Scheithauer |author42=A. Sternberg |author43=O. Straub |author44=C. Straubmeier |author45=E. Sturm |author46=L. J. Tacconi |author47=F. Vincent |author48=S. von Fellenberg |author49=I. Waisberg |author50=F. Widmann |author51=E. Wieprecht |author52=E. Wiezorrek |author53=J. Woillez |author54=S. Yazici |display-authors=6 |collaboration=The GRAVITY collaboration|date=April 2019|title=A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty|journal=Astronomy & Astrophysics|volume=625|page=L10|doi=10.1051/0004-6361/201935656|url=https://www.aanda.org/articles/aa/full_html/2019/05/aa35656-19/aa35656-19.html|arxiv=1904.05721|bibcode=2019A&A...625L..10G|s2cid=119190574}}

An accurate determination of the distance to the Galactic Center as established from variable stars (e.g. RR Lyrae variables) or standard candles (e.g. red-clump stars) is hindered by numerous effects, which include: an ambiguous reddening law; a bias for smaller values of the distance to the Galactic Center because of a preferential sampling of stars toward the near side of the Galactic bulge owing to interstellar extinction; and an uncertainty in characterizing how a mean distance to a group of variable stars found in the direction of the Galactic bulge relates to the distance to the Galactic Center.{{cite journal |last=Majaess |first=D |date=March 2010 |title=Concerning the Distance to the Center of the Milky Way and Its Structure |journal=Acta Astronomica |volume=60 |issue=1 |pages=55–74 |bibcode=2010AcA....60...55M |arxiv=1002.2743}}{{cite web |url=http://universeataglance.blogspot.ca/2011/04/milky-way-distance-to-galactic-centre.html |title=Milky Way: Distance to the Galactic Centre |last=Vovk |first=Olga |date=27 April 2011 |website=Universe at a glance (blog) |access-date=2019-03-23}}

The nature of the Milky Way's bar, which extends across the Galactic Center, is also actively debated, with estimates for its half-length and orientation spanning between 1–5 kpc (short or a long bar) and 10–50°.{{cite journal |last1=Cabrera-Lavers |first1=A. |last2=González-Fernández |first2=C. |last3=Garzón |first3=F. |last4=Hammersley |first4=P. L. |last5=López-CorredoiRA |first5=M. |s2cid=15040792 |date=December 2008 |title=The long Galactic bar as seen by UKIDSS Galactic plane survey |journal=Astronomy and Astrophysics |volume=491 |issue=3 |pages=781–787 |bibcode=2008A&A...491..781C |doi=10.1051/0004-6361:200810720 |arxiv=0809.3174}} Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.{{cite journal |last1=Nishiyama |first1=Shogo |last2=Nagata |first2=Tetsuya |last3=Baba |first3=Daisuke |last4=Haba |first4=Yasuaki |last5=Kadowaki |first5=Ryota |last6=Kato |first6=Daisuke |last7=Kurita |first7=Mikio |last8=Nagashima |first8=Chie |last9=Nagayama |first9=Takahiro |last10=Murai |first10=Yuka |last11=Nakajima |first11=Yasushi |last12=TamuRA |first12=Motohide |last13=Nakaya |first13=Hidehiko |last14=Sugitani |first14=Koji |last15=Naoi |first15=Takahiro |last16=Matsunaga |first16=Noriyuki |last17=Tanabé |first17=Toshihiko |last18=Kusakabe |first18=Nobuhiko |last19=Sato |first19=Shuji |s2cid=399710 |date=March 2005 |title=A Distinct Structure inside the Galactic Bar |journal=The Astrophysical Journal |volume=621 |issue=2 |pages=L105–L108 |bibcode=2005ApJ...621L.105N |arxiv=astro-ph/0502058 |doi=10.1086/429291}} The bar is delineated by red-clump stars (see also red giant); however, RR Lyrae variables do not trace a prominent Galactic bar.{{cite journal |last1=Alcock |first1=C. |last2=Allsman |first2=R. A. |last3=Alves |first3=D. R. |last4=Axelrod |first4=T. S. |last5=Becker |first5=A. C. |last6=Basu |first6=A. |last7=Baskett |first7=L. |last8=Bennett |first8=D. P. |last9=Cook |first9=K. H. |last10=Freeman |first10=K. C. |last11=Griest |first11=K. |last12=Guern |first12=J. A. |last13=Lehner |first13=M. J. |last14=Marshall |first14=S. L. |last15=Minniti |first15=D. |last16=Peterson |first16=B. A. |last17=Pratt |first17=M. R. |last18=Quinn |first18=P. J. |last19=Rodgers |first19=A. W. |last20=Stubbs |first20=C. W. |last21=Sutherland |first21=W. |last22=Vandehei |first22=T. |last23=Welch |first23=D. L. |s2cid=16244436 |date=January 1998 |title=The RR Lyrae Population of the Galactic Bulge from the MACHO Database: Mean Colors and Magnitudes |journal=The Astrophysical Journal |volume=492 |issue=1 |pages=190–199 |bibcode=1998ApJ...492..190A |doi=10.1086/305017 |arxiv=astro-ph/9706292}}{{cite journal |last1=Kunder |first1=Andrea |last2=Chaboyer |first2=Brian |s2cid=16046532 |date=December 2008 |title=Metallicity Analysis of MACHO Galactic Bulge RR0 Lyrae Stars from their Light Curves |journal=The Astronomical Journal |volume=136 |issue=6 |pages=2441–2452 |bibcode=2008AJ....136.2441K |doi=10.1088/0004-6256/136/6/2441 |arxiv=0809.1645}} The bar may be surrounded by a ring called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the Milky Way, and most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way.{{cite web |author=Staff |date=12 September 2005 |url=http://www.bu.edu/galacticring/new_introduction.htm |title=Introduction: Galactic Ring Survey |publisher=Boston University |access-date=2007-05-10}}

Supermassive black hole

File:EHT Saggitarius A black hole.tif Sagittarius A*, imaged by the Event Horizon Telescope{{cite news |title=Astronomers reveal first image of the black hole at the heart of our galaxy |url=https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy |access-date=2022-05-12 |newspaper=Event Horizon Telescope |url-status=live |archive-date=2022-05-12 |archive-url=https://web.archive.org/web/20220512132514/https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy }}]]

The complex astronomical radio source Sagittarius A appears to be located almost exactly at the Galactic Center and contains an intense compact radio source, Sagittarius A*, which coincides with a supermassive black hole at the center of the Milky Way. Accretion of gas onto the black hole, probably involving an accretion disk around it, would release energy to power the radio source, itself much larger than the black hole.

A study in 2008 which linked radio telescopes in Hawaii, Arizona and California (Very-long-baseline interferometry) measured the diameter of Sagittarius A* to be 44 million kilometers (0.3 AU).{{cite journal |last=Doeleman |first=Sheperd S. |s2cid=4424735 |display-authors=etal |date=2008 |title=Event-horizon-scale structure in the supermassive black hole candidate at that Galactic Centre |journal=Nature |volume=455 |issue=7209 |pages=78–80 |doi=10.1038/nature07245 |pmid=18769434 |bibcode=2008Natur.455...78D |arxiv=0809.2442}}{{cite journal |last=Reynolds |first=Christopher S. |s2cid=205040663 |date=2008 |title=Bringing black holes into focus |journal=Nature |volume=455 |issue=7209 |pages=39–40 |doi=10.1038/455039a |pmid=18769426 |bibcode=2008Natur.455...39R}} For comparison, the radius of Earth's orbit around the Sun is about 150 million kilometers (1.0 AU), whereas the distance of Mercury from the Sun at closest approach (perihelion) is 46 million kilometers (0.3 AU). Thus, the diameter of the radio source is slightly less than the distance from Mercury to the Sun.

Scientists at the Max Planck Institute for Extraterrestrial Physics in Germany using Chilean telescopes have confirmed the existence of a supermassive black hole at the Galactic Center, on the order of 4.3 million solar masses. Later studies have estimated a mass of 3.7 million{{cite journal |last1=Ghez |first1=A. M. |last2=Salim |first2=S. |last3=Hornstein |first3=S. D. |last4=Tanner |first4=A. |last5=Lu |first5=J. R. |last6=Morris |first6=M. |last7=Becklin |first7=E. E. |last8=Duchene |first8=G. |title=Stellar Orbits around the Galactic Center Black Hole |journal=The Astrophysical Journal |date=2005-02-20 |volume=620 |issue=2 |pages=744–757 |doi=10.1086/427175|arxiv=astro-ph/0306130 |bibcode=2005ApJ...620..744G |s2cid=8656531 }}{{cite journal |last1=Schödel |first1=R. |last2=Ott |first2=T. |last3=Genzel |first3=R. |last4=Hofmann |first4=R. |last5=Lehnert |first5=M. |last6=Eckart |first6=A. |last7=Mouawad |first7=N. |last8=Alexander |first8=T. |last9=Reid |first9=M. J. |last10=Lenzen |first10=R. |last11=Hartung |first11=M. |last12=Lacombe |first12=F. |last13=Rouan |first13=D. |last14=Gendron |first14=E. |last15=Rousset |first15=G. |last16=Lagrange |first16=A.-M. |last17=Brandner |first17=W. |last18=Ageorges |first18=N. |last19=Lidman |first19=C. |last20=Moorwood |first20=A. F. M. |last21=Spyromilio |first21=J. |last22=Hubin |first22=N. |last23=Menten |first23=K. M. |title=A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way |journal=Nature |date=October 2002 |volume=419 |issue=6908 |pages=694–696 |doi=10.1038/nature01121|pmid=12384690 |arxiv=astro-ph/0210426 |bibcode=2002Natur.419..694S |s2cid=4302128 }} or 4.1 million solar masses.

On 5 January 2015, NASA reported observing an X-ray flare 400 times brighter than usual, a record-breaker, from Sagittarius A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers.{{cite web |last1=Chou |first1=Felicia |last2=Anderson |first2=Janet |last3=Watzke |first3=Megan |title=Release 15-001 – NASA's Chandra Detects Record-Breaking Outburst from Milky Way's Black Hole |url=http://www.nasa.gov/press/2015/january/nasa-s-chandra-detects-record-breaking-outburst-from-milky-way-s-black-hole/ |date=5 January 2015 |work=NASA}}

{{Wide image|Center of the Milky Way Galaxy IV – Composite.jpg|800px|There is a supermassive black hole in the bright white area to the right of the center of this wide (scrollable) image. This composite photograph covers about half of a degree.}}

=Gamma- and X-ray emitting Fermi bubbles=

{{Multiple image

| direction = vertical

| width = 300

| header = Galactic gamma- and X-ray bubbles

| image1 = 800 nasa structure renderin2.jpg

| image2 = Gamma-ray Bubbles.ogv

| footer = Gamma- and X-ray bubbles at the Milky Way galaxy center: Top: illustration; Bottom: video.

}}

In November 2010, it was announced that two large elliptical lobe structures of energetic plasma, termed bubbles, which emit gamma- and X-rays, were detected astride the Milky Way galaxy's core.{{cite news |url=https://www.cfa.harvard.edu/news/2010-22 |title=Astronomers Find Giant, Previously Unseen Structure in our Galaxy |publisher=Harvard-Smithsonian Center for Astrophysics |first1=David A. |last1=Aguilar |first2=Christine |last2=Pulliam |date=9 November 2010 |id=Release No. 2010-22}} Termed Fermi or eRosita bubbles,{{Cite journal |last1=Yang |first1=H.-Y. Karen |last2=Ruszkowski |first2=Mateusz |last3=Zweibel |first3=Ellen G. |date=7 March 2022 |title=Fermi and eROSITA bubbles as relics of the past activity of the Galaxy's central black hole |journal=Nature Astronomy |volume=6 |issue=5 |pages=584–591 |issn=2397-3366 |doi=10.1038/s41550-022-01618-x |publisher=Springer Nature|arxiv=2203.02526 |bibcode=2022NatAs...6..584Y |s2cid=247292361 }} they extend up to about 25,000 light years above and below the Galactic Center. The galaxy's diffuse gamma-ray fog hampered prior observations, but the discovery team led by D. Finkbeiner, building on research by G. Dobler, worked around this problem. The 2014 Bruno Rossi Prize went to Tracy Slatyer, Douglas Finkbeiner, and Meng Su "for their discovery, in gamma rays, of the large unanticipated Galactic structure called the Fermi bubbles".{{cite news |author= |date=8 January 2014 |url=http://astronomy.fas.harvard.edu/news/2014-rossi-prize-awarded-harvard-astronomy-professor-douglas-finkbeiner-tracy-slayter |title=2014 Rossi prize awarded to Douglas Finkbeiner, Tracy Slatyer, and Meng Su |publisher=Harvard University}}

The origin of the bubbles is being researched.{{cite journal |last1=Yang |first1=H.-Y. K. |last2=Ruszkowski |first2=M. |last3=Zweibel |first3=E. G. |s2cid=56443272 |date=12 February 2018 |title=Unveiling the Origin of the Fermi Bubbles |journal=Galaxies |volume=6 |issue=29 |page=29 |doi=10.3390/galaxies6010029 |arxiv=1802.03890 |bibcode=2018Galax...6...29Y |doi-access=free}}{{Cite news |last=Liu |first=Jia |date=May 15, 2020 |url=https://phys.org/news/2020-05-reveal-common-fermi-galactic-center.html |title=Researchers reveal common origin of Fermi bubbles and galactic center X-ray outflows |work=Phys.org |publisher=Science X Network}} The bubbles are connected and seemingly coupled, via energy transport, to the galactic core by columnar structures of energetic plasma termed chimneys.{{cite journal |last=Chernyakova |first=Masha |date=20 March 2019 |title=X-ray chimneys in the Galactic Centre |journal=Nature |publisher=Springer Nature Publishing |volume= 567 |issue= 7748 |pages=318–320 |doi=10.1038/d41586-019-00811-9 |pmid=30894730 |bibcode=2019Natur.567..318C |doi-access=free}} In 2020, for the first time, the lobes were seen in visible light{{cite journal |last1=Krishnarao |first1=Dhanesh |last2=Benjamin |first2=Robert A. |last3=Haffner |first3=L. Matthew |title=Discovery of High-velocity Hα Emission in the Direction of the Fermi Bubble |journal=The Astrophysical Journal |date=7 August 2020 |volume=899 |issue=1 |pages=L11 |doi=10.3847/2041-8213/aba8f0 |arxiv=2006.00010 |bibcode=2020ApJ...899L..11K |s2cid=220969030 |doi-access=free }} and optical measurements were made.{{Cite web |title=236th Meeting of the American Astronomical Society |url=https://www.abstractsonline.com/pp8/#!/9052/session/282 |access-date=2020-06-08 |website=www.abstractsonline.com}} By 2022, detailed computer simulations further confirmed that the bubbles were caused by the Sagittarius A* black hole.{{Cite news |author=University of Michigan |date=March 8, 2022 |url=https://phys.org/news/2022-03-massive-center-milky-supermassive-black.html |title=Massive bubbles at center of Milky Way caused by supermassive black hole |work=Phys.org |publisher=Science X Network}}

<span id="paradox of youth in Galactic Center"></span>Stellar population

File:Milky Way Galaxy and a meteor.jpg and a meteor]]

The central cubic parsec around Sagittarius A* contains around 10 million stars.{{cite web |url=http://www.astronomy.ohio-state.edu/~ryden/ast162_7/notes31.html |title=Lecture 31: The Center of Our Galaxy}} Although most of them are old red giant stars, the Galactic Center is also rich in massive stars. More than 100 OB and Wolf–Rayet stars have been identified there so far.{{cite journal |last1=Mauerhan |first1=J. C. |last2=Cotera |first2=A. |last3=Dong |first3=H. |s2cid=20968628 |title=Isolated Wolf–Rayet Stars and O Supergiants in the Galactic Center Region Identified Via Paschen-α Excess |journal=The Astrophysical Journal |date=2010 |doi=10.1088/0004-637X/725/1/188 |volume=725 |issue=1 |pages=188–199 |bibcode=2010ApJ...725..188M |arxiv=1009.2769 |url=http://resolver.caltech.edu/CaltechAUTHORS:20110107-141452282}} They seem to have all been formed in a single star formation event a few million years ago. The existence of these relatively young stars was a surprise to experts, who expected the tidal forces from the central black hole to prevent their formation.{{cite journal |last1=Støstad |first1=M. |last2=Do |first2=T. |last3=Murray |first3=N. |last4=Lu |first4=J.R. |last5=Yelda |first5=S. |last6=Ghez |first6=A. |s2cid=118579717 |title=Mapping the Outer Edge of the Young Stellar Cluster in the Galactic Center |journal=The Astrophysical Journal |date=2015 |doi=10.1088/0004-637X/808/2/106 |volume=808 |issue=2 |page=106 |bibcode=2015ApJ...808..106S |arxiv=1504.07239 }}

This paradox of youth is even stronger for stars that are on very tight orbits around Sagittarius A*, such as S2 and S0-102. The scenarios invoked to explain this formation involve either star formation in a massive star cluster offset from the Galactic Center that would have migrated to its current location once formed, or star formation within a massive, compact gas accretion disk around the central black-hole. Current evidence favors the latter theory, as formation through a large accretion disk is more likely to lead to the observed discrete edge of the young stellar cluster at roughly 0.5 parsec.{{cite journal |last1=Støstad |first1=M. |last2=Do |first2=T. |last3=Murray |first3=N. |last4=Lu |first4=J.R. |last5=Yelda |first5=S. |last6=Ghez |first6=A. |s2cid=118579717 |title=Mapping the Outer Edge of the Young Stellar Cluster in the Galactic Center |journal=The Astrophysical Journal |date=2015 |doi=10.1088/0004-637X/808/2/106 |volume=808 |issue=2 |page=106 |bibcode=2015ApJ...808..106S |arxiv=1504.07239 }} Most of these 100 young, massive stars seem to be concentrated within one or two disks, rather than randomly distributed within the central parsec.{{cite web |url=http://www.astro.ucla.edu/~ghezgroup/gc/ |title=UCLA Galactic Center Group |access-date=9 November 2007 |archive-date=26 June 2017 |archive-url=https://web.archive.org/web/20170626010742/http://www.astro.ucla.edu/~ghezgroup/gc/ }}{{cite web |url=http://www.mpe.mpg.de/ir/GC/ |title=Galactic Center}} This observation however does not allow definite conclusions to be drawn at this point.

Star formation does not seem to be occurring currently at the Galactic Center, although the Circumnuclear Disk of molecular gas that orbits the Galactic Center at two parsecs seems a fairly favorable site for star formation. Work presented in 2002 by Antony Stark and Chris Martin mapping the gas density in a 400-light-year region around the Galactic Center has revealed an accumulating ring with a mass several million times that of the Sun and near the critical density for star formation.

They predict that in approximately 200 million years, there will be an episode of starburst in the Galactic Center, with many stars forming rapidly and undergoing supernovae at a hundred times the current rate. This starburst may also be accompanied by the formation of galactic relativistic jets, as matter falls into the central black hole. It is thought that the Milky Way undergoes a starburst of this sort every 500 million years.

In addition to the paradox of youth, there is a "conundrum of old age" associated with the distribution of the old stars at the Galactic Center. Theoretical models had predicted that the old stars—which far outnumber young stars—should have a steeply-rising density near the black hole, a so-called Bahcall–Wolf cusp. Instead, it was discovered in 2009 that the density of the old stars peaks at a distance of roughly 0.5 parsec from Sgr A*, then falls inward: instead of a dense cluster, there is a "hole", or core, around the black hole.{{Cite journal |last1=Buchholz |first1=R. M. |last2=Schödel |first2=R. |last3=Eckart |first3=A. |s2cid=5221750 |title=Composition of the galactic center star cluster: Population analysis from adaptive optics narrow band spectral energy distributions |journal=Astronomy and Astrophysics |volume=499 |issue=2 |pages=483–501 |date=May 2009 |bibcode=2009A&A...499..483B |doi=10.1051/0004-6361/200811497 |arxiv=0903.2135 }}

Several suggestions have been put forward to explain this puzzling observation, but none is completely satisfactory.{{Cite journal |first=David |last=Merritt |author-link=David Merritt |editor-last=Morris |editor-first=Mark |editor2-last=Wang |editor2-first=Daniel Q. |editor3-last=Yuan |editor3-first=Feng |title=Dynamical Models of the Galactic Center |journal=The Galactic Center: A Window to the Nuclear Environment of Disk Galaxies |volume=439 |arxiv=1001.5435 |bibcode=2011ASPC..439..142M |series=The Galactic Center: A Window on the Nuclear Environment of Disk Galaxies |page=142 |place=San Francisco |date=May 2011 }}{{Cite journal |last=Chown |first=Marcus |author-link=Marcus Chown |title=Something's been eating the stars |journal=New Scientist |volume=207 |issue=2778 |pages=30–33 |date=Sep 2010 |url=https://www.scribd.com/doc/39001703/2-6-1sudiendtsmauei |bibcode=2010NewSc.207...30M |doi=10.1016/S0262-4079(10)62278-6 |access-date=9 September 2017 |archive-date=9 December 2014 |archive-url=https://web.archive.org/web/20141209142348/https://www.scribd.com/doc/39001703/2-6-1sudiendtsmauei |url-status=dead }} For instance, although the black hole would eat stars near it, creating a region of low density, this region would be much smaller than a parsec. Because the observed stars are a fraction of the total number, it is theoretically possible that the overall stellar distribution is different from what is observed, although no plausible models of this sort have been proposed yet.

= Stellar black holes =

The galactic center is suspected to have a large population of stellar mass black holes. There are probably around 25,000 stellar mass black holes in the central parsecs of the galactic center as a result of dynamical friction and migration.{{Cite journal |last=Miralda-Escudé |first=Jordi |last2=Gould |first2=Andrew |date=2000-12-20 |title=A Cluster of Black Holes at the Galactic Center |url=https://iopscience.iop.org/article/10.1086/317837/meta |journal=The Astrophysical Journal |language=en |volume=545 |issue=2 |pages=847 |doi=10.1086/317837 |issn=0004-637X}}{{Cite journal |last=Hailey |first=Charles J. |last2=Mori |first2=Kaya |last3=Bauer |first3=Franz E. |last4=Berkowitz |first4=Michael E. |last5=Hong |first5=Jaesub |last6=Hord |first6=Benjamin J. |date=April 2018 |title=A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy |url=https://www.nature.com/articles/nature25029 |journal=Nature |language=en |volume=556 |issue=7699 |pages=70–73 |doi=10.1038/nature25029 |issn=1476-4687}} Theses black holes has a major effect on the stellar population of the galactic center and the S cluster. They limit the number of massive O-type stars through stellar collisions.{{Cite journal |last=Haas |first=J. |last2=Kroupa |first2=P. |last3=Šubr |first3=L. |last4=Singhal |first4=M. |date=2025-03-01 |title=The star grinder in the Galactic centre - Uncovering the highly compact central stellar-mass black hole cluster |url=https://www.aanda.org/articles/aa/abs/2025/03/aa53324-24/aa53324-24.html |journal=Astronomy & Astrophysics |language=en |volume=695 |pages=L19 |doi=10.1051/0004-6361/202453324 |issn=0004-6361}}

Gallery

In May 2021, NASA published new images of the Galactic Center, based on surveys from Chandra X-ray Observatory and other telescopes.{{cite journal|last1=Wang |first1=Q. Daniel |title=Chandra large-scale mapping of the Galactic Centre: Probing high-energy structures around the central molecular zone |journal=Monthly Notices of the Royal Astronomical Society |year=2021 |volume=504 |issue=2 |pages=1609–1618 |doi=10.1093/mnras/stab801 |doi-access=free |arxiv=2010.02932}} Images are about 2.2 degrees (1,000 light years) across and 4.2 degrees (2,000 light years) long.

{{Multiple image

| align = center

| image1 = Galactic Center Magnetized Threads labeled.jpg

| caption1 = Composite labeled image

| image2 = Galactic Center Magnetized Threads 01.jpg

| caption2 = Composite image

| image3 = Galactic Center Magnetized Threads X-ray & Radio Single Color Composite.jpg

| caption3 = X-ray and Radio single color composite

| image4 = Galactic Center Magnetized Threads Radio Single Color.jpg

| caption4 = Radio single color

| header = A panorama of the Galactic Center builds on previous surveys from Chandra X-ray Observatory and other telescopes. In the first image, X-rays from Chandra are orange, green, and purple, showing different X-ray energies, and the radio data from MeerKAT are gray. The next images show single (broadband) colors, with Chandra X-ray data in pink and MeerKAT radio data in blue.

}}

File:Lights out in the galactic centre.jpg|A small portion of a gigapixel color mosaic of the Milky Way's heart{{cite web|title=Lights out in the galactic centre|url=http://www.eso.org/public/images/potw1818a/|website=www.eso.org|access-date=30 April 2018}}

File:Hubble captures glittering crowded hub of our Milky Way.jpg|Red giant stars coexist with white, Sun-like stars.{{cite web|title=Hubble captures glittering crowded hub of our Milky Way|url=https://spacetelescope.org/images/opo1801a/|website=www.spacetelescope.org|access-date=15 January 2018}}

File:Hubble Spots White Dwarfs in Milky Way's Central Hub.jpg|White Dwarfs in Milky Way's Central Hub{{cite web|title=Hubble Spots White Dwarfs in Milky Way's Central Hub|url=http://www.spacetelescope.org/images/opo1538a/|access-date=9 November 2015}}

File:Center Milky Way.jpg|The center of the Milky Way – image taken by ISAAC, the VLT's near- and mid-infrared spectrometer and camera

File:Milky Way IR Spitzer.jpg|Infrared image from Spitzer Space Telescope

File:Milky way 2 md.jpg|A view of the night sky near Sagittarius, enhanced to show better contrast and detail in the dust lanes. The principal stars in Sagittarius are indicated in red.

File:Centre of the Milky Way.jpg|The central parts of the Milky Way, as observed in the near-infrared with the NACO instrument on ESO's Very Large Telescope

File:An Infrared View of the Galaxy.jpg|Infra-red image of the center of the Milky Way revealing a new population of massive stars

File:X-RayFlare-BlackHole-MilkyWay-20140105.jpg|Detection of an unusually bright X-ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy

File:Space SKA telescope image of Galactic Center.jpg|The center of the Milky Way, as imaged by 64 radio telescopes of the South African MeerKAT array

{{CSS image crop

|Image = Milky_way_map.png

|bSize = 1400

|cWidth = 500

|cHeight = 500

|oTop = 545

|oLeft = 450

|Location = center

|Description = The surroundings of the Galactic Center (top view map)

}}

Notes and references

External links

[http://www.astro.ucla.edu/~ghezgroup/gc/ UCLA Galactic Center Group] {{Webarchive|url=https://web.archive.org/web/20170626010742/http://www.astro.ucla.edu/~ghezgroup/gc/ |date=26 June 2017 }}
[http://www.mpe.mpg.de/ir/GC/ Max Planck Institute for Extraterrestrial Physics Galactic Center Group]
[http://press.princeton.edu/titles/8453.html The Galactic Supermassive Black Hole]
[https://web.archive.org/web/20070706183425/http://www.icpress.co.uk/physics/p394.html The Black Hole at the Center of the Milky Way]
[https://web.archive.org/web/20071031014713/http://www.einstein-online.info/en/spotlights/milkyway_bh/index.html The dark heart of the Milky Way]
[https://www.youtube.com/watch?v=uVlcIb-rClI Animation showing orbits of stars near the center of the Milky Way galaxy]
[https://www.youtube.com/watch?v=XhHUNvEKUY8 Zooming in on the center of the Milky Way]
[https://web.archive.org/web/20020808061115/http://www.space.com/scienceastronomy/astronomy/boom_times_020610-1.html Dramatic Increase in Supernova Explosions Looms]
APOD:
[http://antwrp.gsfc.nasa.gov/apod/ap970121.html Journey to the Center of the Galaxy]
[http://antwrp.gsfc.nasa.gov/apod/ap970501.html A Galactic Cloud of Antimatter]
[http://antwrp.gsfc.nasa.gov/apod/ap001220.html Fast Stars Near the Galactic Center]
[http://antwrp.gsfc.nasa.gov/apod/ap051023.html At the Center of the Milky Way]
[http://antwrp.gsfc.nasa.gov/apod/ap000629.html Galactic Center Starscape]
[http://antwrp.gsfc.nasa.gov/apod/ap100831.html Annotated Galactic Center]
[https://web.archive.org/web/20090307094030/http://www.orbitsimulator.com/gravity/articles/mwblackhole.html A simulation of the stars orbiting the Milky Way's central massive black hole]
[http://xstructure.inr.ac.ru/x-bin/theme3.py?level=2&index1=72093 Galactic Center on arxiv.org]

Category:Galactic Center

Category:Geometric centers

Category:Articles containing video clips